A flow measurement device is intended to measure a flow rate of fluid which is gas or liquid. Here, the flow rate is defined as an amount of fluid which moves through a given cross section in a unit time.
Generally, a flow measurement device is provided, at a flow path through which fluid to be measured flows, with a detection element for measuring a flow rate of the fluid, and measures the flow rate of fluid by using a detection signal from the detection element. Usage of the flow measurement device include, for example, measurement of a flow rate of gas which is used for combustion in a gas meter, a burning appliance, an internal combustion engine of an automobile and the like, measurement of flow rates of expiration and inspiration, and a flow rate of blood, in medical instruments, observation of a flow rate of air in industrial equipment, and observation of filter clogging in various equipment.
Generally, flow measurement devices are classified as a straight tube type and a diverting type in view of their structures. FIG. 17A illustrates a basic structure of a flow measurement device of straight tube type, and FIGS. 17B and 17C illustrates basic structures of a flow measurement device of diverting type.
As illustrated in FIG. 17A, a flow measurement device of straight tube type 200 has a structure in which a detection element 202 for measuring a flow rate of fluid is directly disposed at a conduit 201 through which the fluid to be measured flows. Generally, in the flow measurement device 200 of straight tube type, the flow velocity of the fluid needs to be lowered to a detectable range of the detection element 202. Therefore, the area of cross section in the conduit 201 needs to be enlarged so that the flow measurement device 200 is made to be enlarged.
Accordingly, when fluid of large flow rate is to be measured, a flow measurement device of diverting type is used. The flow measurement device of diverting type is configured to divert fluid to be measured, and measure a flow rate of the diverted fluid so as to estimate an entire flow rate (for example, refer to Patent Documents 1 to 5).
Specifically, in a flow measurement device 210 illustrated in FIG. 17B, a resistive element 211 for providing a suitable resistance to the flowing fluid is disposed in the conduit 201. Moreover, an inlet 213 and an outlet 214 of a diverter 212, through which a diversion of the fluid flows, are connected to the upstream side and the downstream side of the resistive element 212 respectively, at the peripheral section of the conduit 201. Then, the detection element 202 is disposed at the diverter 212. According to the above-mentioned constitution, the resistive element 211 provides a suitable resistance to the fluid, which flows through the conduit 201. With this, a part of the fluid is caused to flow into the inlet 213 of the diverter 212.
On the other hand, a flow measurement device 220 of diverting type illustrated in FIG. 17C is configured in such a way that a diverter 222, through which a diversion of the fluid flows, is disposed at the central section of the conduit 201, and the detection element 202 is disposed at this diverter 222. In this case, the diverter itself functions as a resistive element which disturbs the flow of fluid in the conduit 201 so that the fluid is made to flow into the diverter 222.
Patent Document 1: Japanese translation of PCT international application No. 2003-523506 (published on Aug. 5, 2003)
Patent Document 2: Japanese Unexamined Patent Publication No. 11-166720 (published on Jun. 22, 1999)
Patent Document 3: Japanese Unexamined Patent Publication No. 2006-329927 (published on Dec. 7, 2006)
Patent Document 4: Japanese Unexamined Patent Publication No. 2006-308518 (published on Nov. 9, 2006)
Patent Document 5: Japanese Unexamined Patent Publication No. 2010-060287 (published on Mar. 18, 2010)